1. Field of the Invention
The invention relates to a ball bearing.
2. Description of the Related Art
A conventional ball bearing is described in, for example, Japanese Patent Application Publication No. 2004-340277. The ball bearing includes an outer ring, an inner ring, a plurality of balls and a cage. The balls are disposed between an inner periphery raceway groove of the outer ring and an outer periphery raceway groove of the inner ring, are held by the cage, and are located at intervals in the circumferential direction.
The aforementioned cage is a snap cage having only one annular portion and a plurality of cage bars. The cage bars extend from one-side axial end portion of the annular portion toward the one side in the axial direction. The cage bars are arranged at intervals in the circumferential direction. The cage has pockets defined between the cage bars that are adjacent to each other in the circumferential direction.
The balls are disposed in the respective pockets. The distal end portions of the cage bars are curved so as to narrow openings of the pockets. Thus, the balls are prevented from coming out of the pockets toward the one side in the axial direction. The cage is disposed so as to be apart from both the outer ring and the inner ring, and is in contact with only the balls. The cage is held and guided by the balls, so that the cage is prevented from coming off the ball bearing.
In the conventional bearing, the balls need to be held by curved faces of the cage, which define the pockets, in order to prevent the balls and the cage from being separated from each other. Thus, the area of contact between the cage and the balls becomes larger, and therefore the fictional torque due to friction between the cage and the balls and the agitation torque based on agitation resistance induced in grease disposed between the cage and the balls are increased. As a result, the torque becomes larger, and the operation costs are increased.
Further, the large agitation resistance increases the degree of temperature rise, and thus the service life of the grease is shortened. Due to problems such as reduction in the service life of the grease caused by a high degree of temperature rise, the conventional ball bearings are not suitable for use with a high speed rotation.